Studio della propagazione della frattura in polibutene per tubi

Il Polibutene-1 isotattico (i-PB1) è un materiale polimerico usato per la produzione di tubi per il trasporto di fluidi in pressione. In questo lavoro si sono studiati due tipi di i-PB1 prodotti da Basell che differiscono per grado di isotatticità. Si sono condotte prove di frattura a diverse temperature e velocità di spostamento imposte. Si è utilizzata una configurazione di flessione su provini con singolo intaglio (SENB) unitamente a quella di doppia trave a sbalzo (DCB), quest’ultima limitatamente allo studio della fase di propagazione. Al fine di individuare con precisione l’innesco della frattura e la velocità di propagazione della stessa si è fatto ricorso a metodi ottici. Dal punto di vista fenomenologico durante la propagazione si assiste alla formazione di zone in cui il materiale risulta fortemente stirato. La frattura in esse avanza con una lacerazione continua che si alterna a salti repentini in occasione del brusco cedimento di queste zone, associato a conseguenti cadute del carico. Questa parziale instabilità è stata osservata sui due materiali per entrambe le configurazioni di prova. I risultati ottenuti sono stati interpretati seguendo l’approccio della meccanica della frattura e applicando uno schema di riduzione di tipo tempo-temperatura che ha permesso di descrivere il comportamento viscoelastico del materiale su un intervallo temporale di diverse decadi. I risultati hanno permesso di applicare un modello analitico per la previsione della vita utile di tubi in pressione. Il modello si è mostrato in buon accordo con i dati sperimentali disponibili da prove condotte su tubi dello stesso materiale

Towards Safer Helmets: Characterisation, Modelling and Monitoring

Bike and ski helmets are mainly made up of two layers: the external shell and the foam liner. The foam liner, typically made of expanded polystyrene (EPS) or polypropylene (EPP), is asked to provide energy absorption in case of impacts. Standard helmet design requires the foam to maximize this energy absorption, thus achieving large deformations (up to 25% in compression) while maintaining the stress level below a threshold value. To optimize the helmet construction in terms of foam composition, structure and density, reliable numerical models are required, which in turn need to be fed with accurate experimental data. A characterisation of several foams was performed, including EPS and EPP having varying densities, under tensile and compressive stress states at varying strain rates. Typical mechanical parameters (elastic moduli and plateau stress in compression, Poisson's ratio) were compared with literature data and applicability of existing models to experimental results was discussed. A marked strain rate dependence – very important for impact applications – was accurately described using the Nagy phenomenological model. The foam microstructure was investigated using scanning electron microscopy (SEM) to assess structural changes before and after compression. The aforementioned mechanical features were then adopted in a rate-dependent constitutive law for crushable foams, to model the shock attenuation properties of helmets and validate the approach against available data. Finally, a microelectromechanical system based in-helmet wireless micro monitoring system was developed and inserted in a helmet prototype. The system is capable of acquiring impact load spectra, providing valuable information to investigate generic impacts with varying angles and energy. In particular, it can monitor the effect of repeated micro-impacts on the residual energy absorption characteristics of the foam

Fracture of Polyjet 3D printed materials: a preliminary investigation

Additive manufacturing (AM), in particular 3D printing, gained a lot of interest in the past few years. This work is focused in particular on the Polyjet 3D process by means of which photo-curable polymers with strongly different physical and mechanical properties can be injected (in the form of liquid droplets) and cured through the use of a UV lamp. In previous works [1,2] we already highlighted the important influence that the interphase between different constituents can have on the viscoelastic properties of the 3D printed composite materials. In view of extending our research beyond small deformations and towards the determination of the fracture properties of Polyjet composites, a preliminary investigation was carried out to characterize the fracture behaviour of base constituents, and to verify the applicability of conventional fracture mechanics approaches to this particular class of AM materials/structures. As a first step, the effect of several parameters on the apparent fracture properties was determined: material composition (rubber content), printing orientation, presence of support material and ageing time. For this study, two polymers were considered: VeroWhitePlus (RGD835) and VeroGray (RGD850). They both share the same glassy matrix, but VeroGray also includes a secondary rubbery phase. Tensile and scratch experiments were performed to evaluate bulk and surface mechanical properties, later to be considered as a basis to analyze fracture data obtained on three point bending notched samples, tested according to ISO 13586 to determine apparent toughness and fracture energy values, KIC and GIC. The applicability of a fracture mechanics framework to these materials was discussed

Influence of morpho-structural parameters on the environmental stress cracking of polyethylene

The environmental stress cracking resistance (ESCR) of four polyethylenes in active medium (10% Tergitol solution) was investigated. The four materials were chosen to explore a broad range of performance and applications; they are listed according to increasing expected ESCR: - an injection moulding, low-molecular weight (MW) HDPE homopolymer - two rotomoulding LLDPE copolymers with a different comonomer (butene or hexene) - a blown film extrusion high MW HDPE copolymer (hexene) The fracture resistance of the hexene LLDPE copolymer was expected to be slightly higher than that of the butene one, because of the longer alkyl group. The two materials were chosen to challenge the ability of the different testing methods to discriminate between similar levels of ESCR. Several analytical techniques were employed to obtain relevant morpho-structural parameters: density, degree of crystallinity, MW distribution, short chain branch content and average lamellar thickness. ESCR was evaluated by employing three well-known but widely different approaches: - the Bell Telephone test (ASTMD1693), performed on notched specimens immersed in the active environment at 50°C - strain hardening modulus (SHM), obtained from tensile tests performed in air at 80°C - fracture mechanics (FM) tests on three- and four-point bending notched specimens, performed at varying temperatures both in air and in the active environment; two different loading histories were considered (creep and constant displacement rate) MW distribution seems to have a greater effect on ESCR behaviour with respect to other morpho-structural parameters. There was instead a strong consistency of the different ESCR testing methods, so that SHM could replace ASTMD1693 as an industrial test aimed at ranking ESCR of polyethylenes. FM, while being more complex to carry out, provides a wealth of additional information which could be used to actually predict the lifetime of products working in an ESC environment

Modeling of shock absorption in athletics track surfaces

In this work, the possibility of predicting the force reduction (FR) characterizing the shock absorption capability of track surfaces by finite element modeling was investigated. The mechanical responses of a typical sport surface and of a reference material were characterized by quasi-static uniaxial compression experiments and fitted by Neo-Hookean and Mooney–Rivlin’s hyperelastic models to select the more appropriate one. Furthermore, in order to examine the materials behavior at strain rates typical of athletics applications, the rate dependence of the constitutive parameters was investigated. A finite element model, taking into consideration the post-impact nonlinear dynamics of the track surface and of the system (track surface + artificial athlete), was developed and validated through comparison with the results of FR tests. The simulations showed a very good agreement with the experiments and allowed to interpret the experimentally observed combined effect of track thickness and material intrinsic properties on the overall surface behavior

A 3D Numerical Model for the Optimization of Running Tracks Performance

In previous works, a finite element model of the shock absorbing characteristics of athletics tracks was developed, able to give sufficiently reliable predictions from laboratory tests performed on suitable material samples. The model proved to be effective in discriminating the effects of geometry (i.e. thickness) and material properties (essentially the elastic characteristics) on force reduction, thus allowing a first optimization of the tracks in view of their approval by the International Association of Athletics Federations (IAAF). This simplified 2D model neglected the real track structure, considering it as a single layer of material having homogenized properties. In the present study, a new 3D model was developed to accurately describe the structure of multi-layered tracks, with properties and geometrical construction (e.g. solid or honeycomb) differing from one layer to another. Several tracks having different combinations of top/bottom layers varying in both material formulation (i.e. chemical composition) and geometry were thus considered. Mechanical properties of the individual elements constituting the track were measured with small-scale laboratory tests, taking into account their strain-rate dependence. The 3D model allowed a complete representation of the loads acting on the track and it gave results which are in very good agreement with the experiments. This proves it to be a valuable tool for the purpose of optimizing the track in terms of material formulation as well as layer geometrical construction and arrangement: as an example, the effect of changing the cell size of the honeycomb pattern was investigated

Analysis of ball milling time to produce self-lubricating copper-tungsten disulfide composite: best trade-off between tribological performance and electrical properties

Ball milling is a fundamental step of powder metallurgy widely employed for composite manufacturing. This work focuses on the influence of ball milling time on the morphological, electrical, and tribological properties of self-lubricating copper-tungsten disulfide (Cu-WS2) composites. The study investigates ball milling times between 1 and 24 h to guarantee different degrees of incorporation of WS2 in the copper matrix. Micro-scratch and wear tests are performed to evaluate the tribological behavior. Optical, scanning electron, and confocal laser scanning microscopy analyze the scratch and wear tracks. The results show the reliability of the production process and a general improvement of the composites’ mechanical properties compared to pure copper. The addition of WS2 enhances the tribo-mechanical properties, increasing hardness and wear resistance and decreasing the friction coefficient. Shorter ball milling times result in larger WS2 flakes distributed in the copper matrix, while longer ball milling times result in smaller and more dispersed particles. This homogeneous fine dispersion determines a difference in the composites’ electrical conductivity and tribological performance, with shorter ball milling times (i.e., between 2 and 4 h) offering the best trade-off between wear behavior and electrical properties

Polymeric foams 3D numerical mechanical modelling

One of the main open issues in the field of polymeric foam materials is the lack of a relationship between the foam geometrical characteristics and its constituent material properties, on one side, and the macroscopic mechanical behavior. This link is an essential ingredient for the development of a predictive numerical model able to fully describe the mechanical behavior of polymeric foams under different loading conditions, which is the ultimate goal of the present work. In order to build up a systematic and methodological approach to this problem, polymeric structural closed cell foams having different nominal densities (ranging from 60 to 120 kg/m3) were considered. The internal foam structure was investigated throughout micro-Computed Tomography; the acquired stack of images were processed with a home-made algorithm in which Mean Intercept Length method was implemented to compute material volume distribution and the degree of structural anisotropy. The algorithm also allowed the reconstruction of the real geometry using a voxel-based scheme, to perform Finite Element Analysis. With the aim of reducing geometric discontinuities, inherent in the reconstructed voxel mesh, Taubin’s smoothing algorithm was employed to obtain more accurate results. Numerical simulations mimicking experimental quasi-static uniaxial compression test were performed to obtain nominal stress vs. strain curves. To this purpose, suitable mechanical properties were identified for the (equivalent solid) constituent material: the resulting constitutive law highlights the contribution of the material to the macroscopic foam properties. Relevant mechanical parameters such as elastic moduli, buckling strain and plateau stress were then evaluated and related to geometrical features of the real foam

Delamination of copper/moulding compound interfaces in microelectronics packaging

A crucial aspect influencing the reliability of electronic devices is delamination occurring at the interface between different layers made of dissimilar materials. This type of failure is typically prompted by mechanical stresses arising because of the strong thermal gradients and a large mismatch in the thermal expansion coefficient of the materials involved. Device design may be greatly improved by providing reliable models, whose main challenge is the identification of suitable input parameters. In the present work we focused on the interface between copper and two different polymeric moulding compounds (MCs) in a power semiconductor package. MC is a composite material made of epoxy resin embedding a large volume fraction of silica micro-particles. A combined experimental/numerical method was developed to describe delamination. The surface and bulk mechanical properties of the constituents were first characterized experimentally by performing tensile, dynamic-mechanical analysis and scratch tests in the temperature range of interest. In the next step, bi-material laminae were tested using a fixed-arm peeling configuration at varying angles (45°, 90°, 135°). During testing extensive plastic deformation of the thinner Cu adherend occurred, whose contribution to the experimentally measured peeling force was dominant with respect to the interfacial fracture energy. A global energy balance approach, based on the testing protocol proposed by ESIS TC4 to become an ISO standard, was adopted to obtain the fracture energy associated with the peeling process, using the publicly available ICPeel software together with the previously obtained experimental data. As a result, consistent values for the fracture energy of both copper/MC interfaces were identified. These data were used to calibrate a 2D cohesive zone model implemented in the commercial finite element code Abaqus. The resulting FE model was successfully validated by comparing its predictions with additional fracture experiments performed on the same bimaterial laminae using a notched four-point bending configuration

Bee communities (Hymenoptera: Anthophila) of the "Cerrado" ecosystem in Sao Paulo State, Brazil

Five surveys of the bee communities in four "Cerrado" ecosystem reserves in Sao Paulo State were compared for species richness and similarity. These areas are fragment vegetation reser-reserves located in the Cerrado Corumbata Reserve (Corumbata), Jata Ecological Park (Luiz Antonio), Cajuru (Cajuru), and Vassununga State Park - ""Gleba de Cerrado de Pe-de-Gigante"" (Santa Rita do Passa Quatro). The methodology consisted of capturing bees foraging on flowers along transects, though with small differences between surveys. These ""cerrado"" areas have a large number of species of native bees, which are important pollinators in several Brazilian ecosystems. The community of bees varied among these different fragments. Based on 500 individuals (standardized by rarefaction), Cajuru, Corumbata 1 and Corumbata 2 were the areas with highest species richness, and Jata and Pe-de-Gigante had the lowest species richness in the bee communities. The bee faunas of Corumbata 2 and Pe-de-Gigante had the highest similarity, forming a group with the bee fauna of Cajuru. The bee faunas of Corumbata 1 and Jata were isolated from this group. We found that the bee species richness and similarity found in these ""cerrado"" areas cannot be explained by general factors such as the size of the fragment, the species richness of plants and the distance between the areas. Therefore, we suppose that local factors that differ among areas, such as interactions between populations, and competition and interference from surrounding areas influence and determine bee species richness and similarity in these reserves.FAPESP (Funda ao de Amparo a Pesquisa do Estado de Sao Paulo)[00/06405-2]CNP